The influence of design parameters on the FEA-determined stress distribution in CAD–CAM produced all-ceramic dental crowns

Effect of intermediate ceramics and firing temperature on bond strength between tetragonal zirconia polycrystal and veneering ceramics

The purpose of the present study was to investigate the influence of the intermediate ceramics and firing temperature on bond strength between tetragonal zirconia polycrystal (TZP) and its intermediate ceramics. Two types of intermediate ceramics, defined as a ceramics placed between the TZP and its veneering ceramics, were used; one including high-strength lithium-disilicate (EP) or feldspathic liner porcelain (SB). The firing temperature of the intermediate ceramics was set at 930°C, 945°C or 960°C. Shear bond strength showed values of 35.8 MPa in SB and 54.9 MPa in EP at a firing temperature of 960°C. Electron probe microanalysis revealed that components of the intermediate ceramics remained on the TZP surface after debonding, indicating that fractures occurred in the intermediate ceramics near the TZP. These results indicate that the bond strength between and a TZP framework and its veneering ceramics could be improved by using a high-strength intermediate ceramics and a comparatively high firing temperature.

Influence of resin cement polymerization shrinkage on stresses in porcelain crowns

OBJECTIVE

The aim of this study was to analyze the influence of polymerization shrinkage of the cement layer on stresses within feldspathic ceramic crowns, using experimentally validated FEA models for (1) increasing occlusal cement thickness; and, (2) bonded versus non-bonded ceramic-cement interfaces.

METHODS

2-D axial symmetric models simulated stylized feldspathic crowns (1.5mm occlusal thickness) cemented with resin-cement layers of 50-500μm on dentin preparations, being loaded (500N) or not. Ceramic-cement interface was either bonded or not. Cement was bonded to the dentin in all models. Maximum axial shrinkage of 0%, 1%, 2%, 3%, 4% and 4.65% were simulated. The first principal stresses developing in the cementation surface at the center and at the occluso-axial line-angle of the crown were registered.

RESULTS

Polymerization shrinkage of the cement increased tensile stresses in the ceramic, especially in loaded non-bonded crowns for thicker cement layers. Stresses in loaded non-bonded crowns increased as much as 87% when cement shrinkage increased from 0% to 4.65% (100-187MPa), for a 500μm-thick cement. Increasing polymerization shrinkage strain raised the tensile stresses, especially at the internal occlusal-axial line-angle, for bonded crowns.

SIGNIFICANCE

Changes in the polymerization shrinkage strain (from 0% to 4.65%) have little effect on the tensile stresses generated at the cementation surface of the ceramic crowns, when the occlusal cement thickness is thin (approx. 50μm for bonded crowns). However, as the cement becomes thicker stresses within the ceramic become significant.

Load-bearing properties of minimal-invasive monolithic lithium disilicate and zirconia occlusal onlays: finite element and theoretical analyses

OBJECTIVE

The aim of this study was to test the hypothesis that monolithic lithium disilicate glass-ceramic occlusal onlay can exhibit a load-bearing capacity that approaches monolithic zirconia, due to a smaller elastic modulus mismatch between the lithium disilicate and its supporting tooth structure relative to zirconia.

METHODS

Ceramic occlusal onlays of various thicknesses cemented to either enamel or dentin were considered. Occlusal load was applied through an enamel-like deformable indenter or a control rigid indenter. Flexural tensile stress at the ceramic intaglio (cementation) surface-a cause for bulk fracture of occlusal onlays-was rigorously analyzed using finite element analysis and classical plate-on-foundation theory.

RESULTS

When bonded to enamel (supported by dentin), the load-bearing capacity of lithium disilicate can approach 75% of that of zirconia, despite the flexural strength of lithium disilicate (400MPa) being merely 40% of zirconia (1000MPa). When bonded to dentin (with the enamel completely removed), the load-bearing capacity of lithium disilicate is about 57% of zirconia, still significantly higher than the anticipated value based on its strength. Both ceramics show slightly higher load-bearing capacity when loaded with a deformable indenter (enamel, glass-ceramic, or porcelain) rather than a rigid indenter.

SIGNIFICANCE

When supported by enamel, the load-bearing property of minimally invasive lithium disilicate occlusal onlays (0.6-1.4mm thick) can exceed 70% of that of zirconia. Additionally, a relatively weak dependence of fracture load on restoration thickness indicates that a 1.2mm thin lithium disilicate onlay can be as fracture resistant as its 1.6mm counterpart.

Influence of substructure design, veneer application technique, and firing regime on the in vitro performance of molar zirconia crowns

OBJECTIVES

The aim of this in vitro study was to evaluate the influence of substructure design, veneer application technique, and firing regime on the failure and fracture resistance of molar zirconia crowns.

METHODS

Six groups (n=8/group) of zirconia crowns were fabricated in simple core (SC) or anatomically reduced (AR) design, veneered with different feldspathic or glass ceramic materials, and defined according to the application technique and firing regime (LT: layering technique; LT_L: LT with long-term cooling; PT: press technique; DV: digital veneering technique). The following groups were investigated: SCLT, ARLT, SCLT_L, SCPT, ARPT, ARDV. Crowns were adhesively bonded to polymethylmethacrylate abutment teeth and subjected to thermal cycling (TC: 2×3000×5°/55°) and mechanical loading (ML: 1.2×10(6); 50N; 1.6Hz) in a chewing simulator with metal-ceramic molar crowns as antagonists. Failures were monitored and fracture resistance determined after aging. Data were statistically analyzed (one-way analysis of variance, ANOVA; post hoc Bonferroni, α=0.05). Crowns were subjected to scanning electron microscopy for fractographic failure analysis.

RESULTS

Failures (chipping, cracks) during TCML were observed in groups SCLT (2×), ARDV (2×) and SCLT_L (1×). Defect sizes varied between 3.5mm (SCLT: crack) and 30.0mm(2) (SCLT_L: chipping). Mean (SD) fracture forces ranged between 1529.0 (405.2)N for SCPT and 2372.3 (351.8)N for ARDV.

SIGNIFICANCE

The failure frequency of veneered zirconia crowns could be reduced by using anatomically reduced substructures, the press veneering technique, and an adapted cooling protocol. Fracture resistance increased with use of anatomically reduced substructures and the digital veneering technique.

Comparison of shear bond strength of two veneering ceramics to zirconia

BACKGROUND

Chip-off fracture of veneering porcelain has been described as the most frequent reason for the failure of zirconia-based fixed partial dentures. The purpose of this study was to evaluate the shear bond strength (SBS) of two commercial zirconia core ceramics to their corresponding veneering ceramics.

MATERIALS AND METHODS

Zirconia disks with 7-mm diameter and 3-mm height were prepared (Cercon and Biodenta systems) and veneered with recommended layering ceramics (Cercon ceram and 2 in 1 ceramic, respectively) (n = 10). The disks were polished with diamond paste and airborne-particle abraded before layering. The specimens were mounted in a T-shaped metal holder using autopolymerized acrylic resin and stored in 37°C distilled water for one week, after which they were subjected to thermal cycling. SBS of zirconia core to veneering ceramic was measured using a universal testing machine and failure modes were determined microscopically. Data were analyzed using t test (α < 0.05).

RESULTS

Mean (±SD) SBS values were 27.19(±3.43) and 28.22(±4.08) MPa for Cercon and Biodenta systems, respectively, with no significant difference. Biodenta system showed more adhesive failure compared to more combined (adhesive and cohesive) failures in Cercon system.

CONCLUSION

Within the limitations of this study it can be concluded that SBS of Biodenta and Cercon specimens were nearly the same, but the fracture mode of these two systems were different. Since Biodenta fracture pattern was predominantly adhesive, it seems that maybe Biodenta porcelain was stronger than Cercon porcelain where as its adhesive bond was weaker.

Three-dimensional finite element modelling of all-ceramic restorations based on micro-CT

OBJECTIVES

To describe and apply a method of modelling dental crowns and three-unit fixed partial dentures (FPD) for finite element analyses (FEA) from 3D images obtained using a micro-CT scanner.

METHODS

A crown and a three-unit fixed partial denture (FPD) made of a ceramic framework (Y-TZP) and veneered with porcelain (VM9) were scanned using an X-ray micro-CT scanner with a pixel size of 6.97 μm. Slice images from both structures were generated at each 0.034 mm and processed by an interactive image control system (Mimics). Different masks of abutments, framework and veneer were extracted using thresholding and region growing tools based on X-ray image brightness and contrast. 3D objects of each model were incorporated into non-manifold assembly and meshed simultaneously. Volume meshes were exported to the FEA software (ABAQUS), and the load-generated stress distribution was analyzed.

RESULTS

FEA models showed great shape resemblance with the structures. The use of non-manifold assembly ensured matching surfaces and coinciding nodes between different structural parts. For the crown model, tensile stresses were concentrated in the internal surface of the core, near to the applied load. For the FPD model, the highest tensile stresses were located in the framework, on the cervical area of connectors and pontic.

CONCLUSIONS

Valid 3D models of dental crown and FPD can be generated by combining micro-CT scanning and Mimics software, emphasizing its importance as design tool in dental research.

CLINICAL SIGNIFICANCE

The 3D FEA method described in this work is an important tool to predict the stress distribution, assisting on structural design of dental restorations.

Computer-aided evaluation of preparations for CAD/CAM-fabricated all-ceramic crowns

OBJECTIVE

The aim of this study was the evaluation of preparations from general dental practitioners for zirconia crowns and their correlation with clinical recommendations using a digital approach.

MATERIAL AND METHOD

Seventy-five datasets of left first upper molars (FDI 16) prepared for single zirconia crowns by general dental practitioners were analyzed using a computer-aided design software (LAVA(TM) Design; 3M ESPE, Seefeld, Germany) and a 3D-inspection software (COMETinspect®plus version 4.5; Steinbichler Optotechnik, Neubeuern, Germany). Evaluated parameters were convergence angle, undercuts, interocclusal reduction, abutment height, and design of preparation margin.

RESULTS

The mean convergence angle was determined to be 26.7°. The convergence angle in the mesiobuccal to distopalatal dimension was significantly the highest (31.7°), and the abutment height showed a mean value of 4.1 mm. Convergence angle and abutment height showed a negative correlation. Seventy-three percent of the evaluated locations revealed a margin design conforming to ceramic restorations. In over 30 % of the cases, the interocclusal reduction was insufficient. Generally, no preparation fulfilled all recommendations. Five (6.66 %) of the preparations fulfilled four criteria, 16 (21.33 %) preparations fulfilled three criteria, 31 (41.33 %) fulfilled two criteria, 17 (22.66 %) preparations fulfilled one criterion, and 6 (8 %) fulfilled no criterion.

CONCLUSIONS

Within the limitations of this study, most general dental practitioners seem to have difficulties fulfilling all clinical recommendations given for the preparation of zirconia crowns. The presented digital approach seems to be a useful method to evaluate the preparation geometry.

CLINICAL RELEVANCE

The correct preparation geometry represents an important prerequisite for the success of all-ceramic full crowns. As preparations clearly need to be improved, the approach presented could be the basis of a future tool to increase preparation quality in practice and education by direct objective feedback.

Influence of surface treatment on bond strength of veneering ceramics fused to zirconia

In all-ceramic restorations involving a zirconia framework, surface treatment of the zirconia surface is required to enhance bonding strength with the veneering ceramics and thus prevent chipping. The purpose of the present study was to investigate the influence of surface roughness and heat treatment of the zirconia and use of liner porcelain on bond strength between veneering ceramics and a zirconia framework. Debonding/crack-initiation strength (τb) was determined according to ISO 9693. No significant difference was observed among conditions, except with use of a liner under heat treatment, which yielded a τb of 26.0±2.9-28.9±1.7 MPa. Electron probe microanalysis revealed that components of the veneering ceramics remained on the zirconia surface after debonding, suggesting that fractures occur in the veneering ceramics and that improving the strength of the veneering ceramics themselves might increase bond strength.

A Photoelastic Assessment of Residual Stresses in Zirconia-Veneer Crowns

Residual stresses within the veneer are linked to the high prevalence of veneer chipping observed in clinical trials of zirconia prostheses. We hypothesized that the thermal mismatch between the zirconia infrastructure and the veneer porcelain, as well as the rate used for cooling zirconia-veneer crowns, would be directly proportional to the magnitude of residual stresses built within the veneer layer. Two porcelains with different coefficients of thermal expansion were used to veneer zirconia copings, to create high or low thermal mismatches. The crowns were cooled according to a fast- or a slow-cooling protocol. The retardation of polarized light waves was used to calculate the residual stress magnitude and distribution across the veneer, according to the photoelasticity principle, in 1.0-mm-thick crown sections. While thermal mismatch was an important factor influencing the maximum stress development in the veneer, cooling rate had a minor role. Curved surfaces were preferential sites for stress concentration regardless of thermal mismatch or cooling rate.

Influence of surface modification techniques on shear bond strength between different zirconia cores and veneering ceramics

PURPOSE

Veneering porcelain might be delaminated from underlying zirconia-based ceramics. The aim of this study was the evaluation of the effect of different surface treatments and type of zirconia (white or colored) on shear bond strength (SBS) of zirconia core and its veneering porcelain.

MATERIALS AND METHODS

Eighty zirconia disks (40 white and 40 colored; 10 mm in diameter and 4 mm thick) were treated with three different mechanical surface conditioning methods (Sandblasting with 110 µm Al₂O₃ particle, grinding, sandblasting and liner application). One group had received no treatment. These disks were veneered with 3 mm thick and 5 mm diameter Cercon Ceram Kiss porcelain and SBS test was conducted (cross-head speed = 1 mm/min). Two and one way ANOVA, Tukey's HSD Past hoc, and T-test were selected to analyzed the data (α=0.05).

RESULTS

In this study, the factor of different types of zirconia ceramics (P=.462) had no significant effect on SBS, but the factors of different surface modification techniques (P=.005) and interaction effect (P=.018) had a significant effect on SBS. Within colored zirconia group, there were no significant differences in mean SBS among the four surface treatment subgroups (P=0.183). Within white zirconia group, "Ground group" exhibited a significantly lower SBS value than "as milled" or control (P=0.001) and liner (P=.05) groups.

CONCLUSION

Type of zirconia did not have any effect on bond strength between zirconia core and veneer ceramic. Surface treatment had different effects on the SBS of the different zirconia types and grinding dramatically decreased the SBS of white zirconia-porcelain.

Finite element analysis of stress concentration in three popular brands of fiber posts systems used for maxillary central incisor teeth

AIMS AND OBJECTIVES

To study the stress concentrations in endodontically treated maxillary central incisor teeth restored with 3 different fiber post systems subjected to various oblique occlusal loads.

MATERIALS AND METHODS

FEM analysis was used to analyze stress concentrations generated in maxillary anterior teeth. Computer aided designing was used to create a 2-D model of an upper central incisor. Post systems analyzed were the DT Light Post (RDT, Bisco), Luscent Anchor (Dentatus) & RelyX (3M-ESPE). The entire design assembly was subjected to analysis by ANSYS for oblique loading forces of 25N, 80N & 125 N RESULTS: The resultant data showed that the RelyX generated the least amount of stress concentration.

CONCLUSIONS

Minimal stress buildups contribute to the longevity of the restorations. Thus RelyX by virtue of judicious stress distribution is the better option for restoration of grossly decayed teeth.

Standards of teeth preparations for anterior resin bonded all-ceramic crowns in private dental practice in Jordan

Objectives

To investigate if general dental practitioners (GDPs) in private practice in Jordan follow universal guidelines for preparation of anterior teeth for resin bonded all-ceramic crowns (RBCs).

Material and Methods

A sample (n=100) of laboratory models containing 208 tooth preparations for IPS Empress and In Ceram, featuring work from different GDPs, was obtained from 8 commercial dental laboratories. Aspects of preparations were quantified and compared with accepted criteria defined following a review of the literature and recommendations of the manufactures' guidelines.

Results

Subgingival margins on the buccal aspect were noticed in 36% of the preparations, 54% demonstrated overpreparation with a tendency to overprepare the teeth on the mesiodistal plane more than buccolingual plane. Twenty percent of samples presented a shoulder finish line while a chamfer margin design was noticed in 39%. Twenty-nine percent and 12% of samples had either a feathered or no clear margin design respectively. Incisal under preparation was observed in 18% of dies of each type. Only 17% of all preparations were found to follow the recommended anatomical labial preparations while 29% of the RBC preparations were found to have the recommended axial convergence angle. In total, 43% of preparations were found to have the recommended depth of the finish line.

Conclusions

It was found that relevant guidelines for RBC preparations were not being fully adhered to in private practice in Jordan.

Using occlusal wear information and finite element analysis to investigate stress distributions in human molars

Simulations based on finite element analysis (FEA) have attracted increasing interest in dentistry and dental anthropology for evaluating the stress and strain distribution in teeth under occlusal loading conditions. Nonetheless, FEA is usually applied without considering changes in contacts between antagonistic teeth during the occlusal power stroke. In this contribution we show how occlusal information can be used to investigate the stress distribution with 3D FEA in lower first molars (M(1)). The antagonistic crowns M(1) and P(2)-M(1) of two dried modern human skulls were scanned by μCT in maximum intercuspation (centric occlusion) contact. A virtual analysis of the occlusal power stroke between M(1) and P(2)-M(1) was carried out in the Occlusal Fingerprint Analyser (OFA) software, and the occlusal trajectory path was recorded, while contact areas per time-step were visualized and quantified. Stress distribution of the M(1) in selected occlusal stages were analyzed in strand7, considering occlusal information taken from OFA results for individual loading direction and loading area. Our FEA results show that the stress pattern changes considerably during the power stroke, suggesting that wear facets have a crucial influence on the distribution of stress on the whole tooth. Grooves and fissures on the occlusal surface are seen as critical locations, as tensile stresses are concentrated at these features. Properly accounting for the power stroke kinematics of occluding teeth results in quite different results (less tensile stresses in the crown) than usual loading scenarios based on parallel forces to the long axis of the tooth. This leads to the conclusion that functional studies considering kinematics of teeth are important to understand biomechanics and interpret morphological adaptation of teeth.

Comparison of chamfer and deep chamfer preparation designs on the fracture resistance of zirconia core restorations

Background and aims

One of the major problems of all-ceramic restorations is their probable fracture under occlusal force. The aim of the present in vitro study was to compare the effect of two marginal designs (chamfer and deep chamfer) on the fracture resistance of all-ceramic restorations, CERCON.

Materials and methods

This in vitro study was carried out with single-blind experimental technique. One stainless steel die with 50’ chamfer finish line design (0.8 mm deep) was prepared using a milling machine. Ten epoxy resin dies were prepared. The same die was retrieved and 50' chamfer was converted into a deep chamfer design (1 mm). Again ten epoxy resin dies were prepared from the deep chamfer die. Zirconia cores with 0.4 mm thickness and 35 µm cement space were fabricated on the epoxy resin dies (10 chamfer and 10 deep chamfer samples). The zirconia cores were cemented on the epoxy resin dies and underwent a fracture test with a universal testing machine and the samples were investigated from the point of view of the origin of the failure.

Results

The mean values of fracture resistance for deep chamfer and chamfer samples were 1426.10±182.60 and 991.75±112.00 N, respectively. Student’s t-test revealed statistically significant differences between the groups.

Conclusion

The results indicated a relationship between the marginal design of zirconia cores and their fracture re-sistance. A deep chamfer margin improved the biomechanical performance of posterior single zirconia crown restorations, which might be attributed to greater thickness and rounded internal angles in deep chamfer margins.

Strain analysis of maxillary complete denture with three-dimensional finite element method

STATEMENT OF PROBLEM

The fracture of maxillary complete dentures has been reported as the most common prosthesis failure.

PURPOSE

The purpose of this study was to evaluate strain distribution in dentures during application of occlusal load with 3-dimensional (3-D) finite element analysis (FEA).

MATERIAL AND METHODS

A maxillary complete denture was converted into a 3-D numerical model by an advanced topometric sensor digitizer (ATOS). The denture surfaces were scanned with fringes. Ten measurements were made for each scan of the denture in top, left, right, back, and front orientations by tilting the scanning table. The individual scans were merged by the digitizing software into a single image. A haptic device with a freeform system (PHANTOM) was used to create the mucosa in contact with the intaglio surface of the denture model. Supporting bone was then constructed from the mucosa model. The posterior teeth were loaded with an occlusal force of 230 N, and the basal bone was constrained for performing FEA.

RESULTS

The highest tensile and compressive strains were found at the incisal and labial frenal notches, respectively. Strains on the intaglio surface of the denture were primarily compressive. The buccal flange exhibited tensile strains in the horizontal direction but compressive strains in the vertical direction. The labial flange showed compressive strains in both directions. The posterior border of the denture flexed away from the mucosa during occlusal loading.

CONCLUSIONS

Three-dimensional FEA provided different views of strain distribution in the denture and indicated that denture failure was unlikely to occur at the shallow labial frenal notch because the strain is compressive. The high tensile strain concentration at the incisal notch is likely to be the cause of denture fracture during clinical service.

Biomechanical Evaluation of an Anatomically Correct All-Ceramic Tooth-Crown System Configuration: Core Layer Multivariate Analysis Incorporating Clinically Relevant Variables

In a crown system, core fracture requires replacement of the restoration. Understanding maximum principal stress concentration in the veneered core of a tooth-crown system as a function of variations in clinically relevant parameters is crucial in the rational design of crown systems. This study evaluated the main and interacting effects of a set of clinical variables on the maximum principal stress (MPS) in the core of an anatomically correct veneer-core-cement-tooth model. A 3D CAD model of a mandibular first molar crown was generated; tooth preparation was modeled by reducing the proximal walls by 1.5 mm and the occlusal surface by 2.0 mm. A cemented veneered core crown was modeled on the preparation. This “crown system” permitted finite element model investigation of the main and interacting effects of proximal wall height reduction, core material, core thickness, cement modulus, cement thickness, and load position on the maximum stress distribution in a factorial design. Analysis of variance was used to identify the main and interacting influences on the level of MPS in the crown core. Statistical significance was set at p<0.05. MPS levels varied as a function of two-way interactions between the following: core thickness and load position; cement thickness and load position; cement modulus and load position; cement thickness and core thickness; and cement thickness and cement modulus; and also three-way interactions among the load position, core material, and proximal wall height reduction, and among the core thickness, cement thickness, and cement modulus. MPS in the crown-tooth system is influenced by the design parameters and also by the interaction among them. Hence, while the geometry of molar crowns is complex, these analyses identify the factors that influence MPS and suggest levels that will minimize the core MPS in future studies of crown design.

Effect of pontic framework design on the fracture resistance of implant-supported all-ceramic fixed partial dentures

Objective:

The purpose of this study was to compare the fracture resistance of implant-supported all-ceramic fixed partial dentures, which have three different pontic designs.

Material and Methods:

Two implants were placed in a metal model simulating mandibular left second premolar and mandibular left second molar. Thirty standardized 3-unit all-ceramic fixed partial dentures with biconvex, convex or concave pontic designs were fabricated using IPS e.max system (n=10). Afterwards, specimens were centrally loaded on the pontics until failure with a universal testing machine. Results were analyzed by Kruskal-Wallis and Mann-Whitney U tests at 5% significance level.

Results:

The fracture resistance values of all-ceramic fixed partial dentures designed with biconvex, convex or concave pontics were 349.71, 438.20 and 300.78 N, respectively. There were no statistically significant differences between the fracture resistances of the groups (p>0.05), except for convex and concave groups (p<0.05 and p=0.009, respectively).

Conclusions:

Convex design showed the best mechanical properties as demonstrated by the high values of fracture resistance.

Design features of a three-dimensional molar crown and related maximum principal stress. A finite element model study

OBJECTIVE

To evaluate the effects of clinically relevant variables on the maximum principal stress (MPS) in the veneer layer of an anatomically correct veneer-core-cement-tooth model.

METHODS

The average dimensions of a mandibular first molar crown were imported into CAD software; a tooth preparation was modeled by reducing the proximal walls by 1.5 mm and the occlusal surface by 2.0 mm. 'Crown systems' were composed by varying characteristics of a cement layer, structural core, and veneer solid, all designed to fit the tooth preparation. The main and interacting effects of proximal wall height reduction, core material, core thickness, cement modulus, cement thickness, and load position on the maximum stress distribution were derived from a series of finite element models and analyzed in a factorial analysis of variance.

RESULTS

The average MPS in the veneer layer over the 64 models was 488 MPa (range = 248-840 MPa). MPS increased significantly with the addition of horizontal load components and with increasing cement thickness. In addition, MPS levels varied as a function of interactions between: proximal wall height reduction and load position; load position and cement thickness; core thickness and cement thickness; cement thickness and proximal wall height reduction; and core thickness, cement thickness and proximal wall height reduction.

CONCLUSION

Rational design of veneered structural ceramics must consider the complex geometry of the crown-tooth system and integrate the influence of both the main effects and interactions among design parameters.

Flexural strength of veneering ceramics for zirconia

OBJECTIVES

The flexural strengths of veneering ceramics for zirconia were compared.

METHODS

With 10 different veneering ceramics for zirconia (test group) and three different veneering ceramics for the metal-ceramic technique (control group) three-point flexural strength and biaxial flexural strength according to ISO 6872: 1995 as well as four-point flexural strength according to EN 843-1: 2005 were measured (n=10). Statistical analysis was performed with one-way ANOVA and post hoc Scheffé test (SPSS, p<0.05).

RESULTS

For the test group, three-point flexural strength ranged between 77.8+/-8.7 and 106.6+/-12.5MPa without any statistically significant differences, biaxial flexural strength between 69.1+/-4.8 and 101.4+/-10.5MPa with three homogeneous groups and four-point flexural strength between 59.5+/-6.2 and 89.2+/-9.5MPa with five homogeneous groups. The control group showed three-point flexural strength values ranging from 93.3+/-13.5 to 149.4+/-20.5MPa, biaxial flexural strength values from 93.4+/-10.0 to 141.2+/-11.6MPa, and four-point flexural strength values from 82.7+/-8.5 to 116.9+/-9.8MPa. In every case, the results of the four-point flexure test were significantly lower than those obtained in the three-point flexure test. The three-point flexural strengths of the test group are similar to those of two ceramics of the control group. The flexural strength of one ceramic of the control group significantly exceeded the strengths of all other ceramics investigated.

CONCLUSION

Three-point flexural strength values of veneering ceramics for zirconia are similar to those of veneering ceramics for the metal-ceramic technique. The four-point flexure test among all three tests showed highest discrimination between the different ceramic materials.

Computer aided analysis of digitized dental stone replicas by dental CAD/CAM technology

OBJECTIVES

To determine the reproducibility of digitized dental stone replicas compared to the master model and the reliability of the computer aided analysis.

METHODS

Four master dies, prepared for complete crowns were fabricated in presintered yttria-stabilized tetragonal zirconia (Y-TZP). Eight vinyl polysiloxane impressions (PROVIL novo; Heraeus Kulzer) were taken of each die and stone replicas were poured in type IV stone (Vel-Mix Stone; Kerr). The master dies and the stone replicas were digitized in a touch-probe scanner (Procera Forte; Nobel Biocare AB), to create triangulated surface-models. The point-cloud from the first of the repeated digitizations of each master die was used as CAD-reference-models (CRM). Discrepancies between the points in the triangulated surface-models and the corresponding CRM were measured by a matching-software (CopyCAD 6.504 SP2; Delcam Plc). The distribution of the discrepancies was analyzed and presented in color-difference-maps.

RESULTS

The precision of the measuring method, presented as the repeatability coefficient, ranged between 7 and 16 microm (entire surface), whereas the analysis of the stone replicas revealed a precision (repeatability coefficient) ranging from 19 to 26 microm. The accuracy of the replica to master (the mean discrepancy) ranged from 0.5 to 2.0 microm (95% confidence interval 1.5-2.9 microm).

SIGNIFICANCE

The greatest precision of the measurement was seen in the jacket surface of the die. The size of the stone replicas varied and the repeatability coefficient was on average 15 microm (2-25 microm) greater for the replica-to-master alignment than the repeated digitizations of the master.

The influence of cavity preparation design on fracture strength and mode of fracture of laboratory-processed composite resin restorations

STATEMENT OF PROBLEM

Removal of large amounts of sound tooth structure may result in a weakened restored tooth. Nevertheless, removal of tooth structure for cuspal coverage has been recommended to protect teeth restored with laboratory-processed composite resin (LPCR) from fracture.

PURPOSE

The purpose of this study was to evaluate the influence of different cavity preparation designs on fracture strength and modes of fracture of teeth restored with LPCR.

MATERIAL AND METHODS

Ninety anatomically similar human third mandibular molars were selected. There were 2 experimental factors, occlusal isthmus width (narrow versus wide) and cuspal coverage (inlay, 1-cusp onlay, 2-cusp onlay, and all-cusp onlay), and 1 control group that received no treatment, resulting in 9 groups (n=10). Indirect composite resin (SR Adoro) restorations were manufactured and adhesively cemented with Adper Single Bond 2 and Rely-X ARC. A compressive loading test (0.5 mm/min) was performed. The modes of fracture were classified according to 4 categories. One-way and 2-way ANOVA followed by Tukey-HSD test were used to statistically analyze the fracture load data (alpha =.05).

RESULTS

The statistical analysis failed to show significant differences among restored groups but showed differences between these groups and the control group (P = .001). Two-way ANOVA failed to show any difference when considering the occlusal isthmus width alone (P = .98), cuspal coverage (P = .273), or the interaction between these factors (P = .972). Several teeth had fractures affecting a great amount of both restoration and tooth structure.

CONCLUSIONS

This in vitro study showed restored teeth having similar fracture strength and fracture modes, suggesting that with the tested preparation designs, there is no advantage of cuspal coverage to protect LPCR restored teeth from fracture.

Effect of loading method on the fracture mechanics of two layered all-ceramic restorative systems

OBJECTIVE

The aim of this research was to evaluate both the fracture and impact strength of two core veneered all-ceramic systems and to reveal whether the speed of loading affects fracture mechanism.

METHODS

The absorbed energy by (IPS)Empress-Eris crowns and Cercon-Ceram S crowns in a fracture strength test was compared by the energy absorbed in an impact strength test. The principles of fractography were used to identify fracture origin and dimensions and to calculate the stress at failure. Finite element analysis (FEA) was used to rationalize the results.

RESULTS

For the (IPS)Empress 2-Eris crowns, there was a significant difference in the energy absorbed for the fracture test and the impact test, where for the Cercon-Ceram S, there was no significant difference. Despite the high strength of the zirconia cores there was no significant difference in the energy absorbed between the two systems in the impact strength test. The dominant mode of failure of layered all-ceramic restorations under occlusal loading is cone cracking in the veneering ceramic.

SIGNIFICANCE

It was concluded that to exploit the high strength of the zirconia cores the strength of the veneering ceramic has to improve as delamination and cone cracking of the veneer are the most expected failure modes.

An overview of treatment considerations for esthetic restorations: a review of the literature

Controversy persists regarding the treatment planning criteria for esthetic restorations. This article reviews the literature regarding the biocompatibility, marginal adaptation, color matching, patient selection, technique sensitivity, and mode and rate of failure of tooth-colored restorations. A Medline search was completed for the period from 1986 to 2006, along with a manual search, to identify pertinent English peer-reviewed articles and textbooks. The key words used were amalgam, posterior composite resin, ceramic inlays/onlays, CEREC, porcelain laminate veneers, all-ceramic crowns, and all-ceramic fixed partial dentures.

Machining Accuracy of Crowns by CAD/CAM System Using TCP/IP: Influence of Restorative Material and Scanning Condition

The purpose of this study was to determine the optimal condition for fabricating accurate crowns efficiently using an internet-based CAD/CAM system. The influences of three different CAD/CAM restorative materials (titanium, porcelain, and composite resin) and three different step-over scanning distances (0.01 mm, 0.11 mm, and 0.21 mm) were evaluated, and their interactive effects were carefully examined. Several points on the inner and outer surfaces of machined crowns - as well as height - were measured. These measurements were then compared with the original models, from which machining accuracy was obtained. At all measuring points, the inner surface of all crowns was machined larger than the die model, whereas the cervical area of porcelain crown was machined smaller than the crown model. Results of this study revealed that a step-over distance of 0.11 mm was an optimal scanning condition, taking into consideration the interactive effects of scanning time required, data volume, and machining accuracy.

The influence of different core material on the FEA-determined stress distribution in dental crowns

OBJECTIVES

All ceramic restorations without metal have great advantages in their biocompatibility and aesthetic aspects. With the introduction of new core materials, the cores are sufficiently strong to produce long lasting all-ceramic restorations; however, the stresses in the veneering porcelain could still determine the longevity. The objective of this study was to evaluate, by finite element analysis (FEA), the influence of different core materials on the stress distribution in dental crowns.

METHODS

The model of a multi-layer all-ceramic crown for posterior tooth 46 produced with CAD-CAM-technology was translated into a three-dimensional FEA program. This crown model was made with gold, zirconia, and alumina-based porcelain core and their matching veneering porcelains. The stress distribution due to the combined influences of bite forces, residual stresses caused by the difference in expansion coefficient of the core material and the veneering porcelain, and the influence of shrinkage of the cement was investigated.

RESULTS

Stiffer core material does not always for various reasons result in lower stresses in the veneering porcelain.

SIGNIFICANCE

This study indicates that the actual distribution of the tensile stresses and the design of restorations must be taken into account; otherwise, the significant contribution of stronger and tougher core materials to the performance of all-ceramic restorations may be offset by the weaker veneering porcelain.